Lipids and Collagen Matrix Restrict the Hydraulic Permeability Within the Porous Compartment of Adult Cortical Bone

In vivo the hydraulic permeability of cortical bone influences the transport of nutrients, waste products and signaling molecules, thus influencing the metabolic functions of osteocytes and osteoblasts. In the current study two hypotheses were tested: the presence of (1) lipids and (2) collagen matrix in the porous compartment of cortical bone restricts its permeability. Our approach was to measure the radial permeability of adult canine cortical bone before and after extracting lipids with acetone-methanol, and before and after digesting collagen with bacterial collagenase. Our results showed that the permeability of adult canine cortical bone was below 4.0 x 10(-17) m(2), a value consistent with prior knowledge. After extracting lipids, permeability increased to a median value of 8.6 x 10(-16) m(2). After further digesting with collagenase, permeability increased to a median value of 1.4 x 10(-14) m(2). We conclude that the presence of both lipids and collagen matrix within the porous compartment of cortical bone restricts its radial permeability. These novel findings suggest that the chemical composition of the tissue matrix within the porous compartment of cortical bone influences the transport and exchange of nutrients and waste products, and possibly influences the metabolic functions of osteocytes and osteoblasts.